Chapter 1

The field of Isotope Geology investigates the isotopic composition of major and trace elements contained in rocks and minerals, with the aim to better understand geological processes. Isotopic techniques are used to address a wide range of geological problems, such as the age of the Earth, the origin and formation of magmatic rocks, palaeotemperatures in sedimentary basins, palaeoclimatology, etc. Isotope geochemistry forms an integral part of modern Earth Sciences and numerous important discoveries have been made thanks to this research. Awareness of these techniques is required to understand research reports and geological interpretations based on isotopic methods. Isotope geochemistry plays an important role in peripheral fields of research such as planetology (origin and evolution of the Solar system) and archaeology (origin and age of settlements, tools and artifacts).

The use of naturally occurring radioactive isotopes to date minerals and rocks is the oldest branch of isotope geology. The foundations of these so-called isotopic or radiometric dating methods were laid shortly after the turn of the XXth century with the discovery of the laws of radioactive decay by eminent physicists such as Ernest Rutherford and Frederick Soddy (Rutherford and Soddy1902a,b). The application of these principles to the field of Geology and the calibration of the geological time scale were pioneered by Arthur Holmes (191119131947). Initially, radiometric geochronology was exclusively based on uranium and its daughter products, but with the development of increasingly sensitive analytical equipment, ever more isotopic ‘clocks’ were added over the course of the century: Rb/Sr (Hahn et al.1943), 14C (Libby1946), K/Ar (Aldrich and Nier1948), 238U fission tracks (Price and Walker1963), 40Ar/39Ar (Merrihue and Turner1966), Sm/Nd (Lugmair1974), etc.

During the 1960s, geochemists began to investigate the non-radiogenic composition of igneous rocks with the aim to understand their source and origin. This line of research greatly expanded over the course of the 1970s and 80s and nowadays the isotopic composition of elements such as Sr and Nd in rocks and minerals is an established petrogenetic indicator. The discovery that the isotopes of the light elements (H, C, N, O, S) are fractionated by physical and chemical processes dates back to the 1930s. The isotopic composition of these elements can therefore be used to detect and understand the hydrospheric and lithospheric processes causing such fractionation (Urey1947). This has led to a better understanding of the physiochemical conditions under which rocks and minerals are formed. Temperature is the most important of these parameters and the aforementioned elements are often used for palaeothermometry.

These lecture notes cover the first half of an Isotope Geology module at UCL that deals with the geochronological aspects of the subject. The second part of the module deals with stable isotopes. It is taught by Dr. Philip Pogge Von Strandmann and covered in a separate set of notes. The core of the geochronology notes is formed by Prof. Peter van den Haute’s lecture notes (in Dutch) at the University of Ghent. This was expanded with additional material, notably on the subjects of cosmogenic nuclide geochronology (Section 8) and U-Th-He dating (Section 7.1). Some figures were modified from published sources, including Allègre (2008), Braun et al. (2006), and Galbraith (2005). These books are recommended further reading material, as is the detailed textbook by Dickin (2005), from which both Allègre (2008) and van den Haute heavily borrowed. Additional lecture material, including the data files used in the programming practicals of Section 12, can be found at